PIC16F886 Pulldown inputs

[Ioannis]

I am sorry but this is not correct as you designed it.

Now the only way is to add the resistors to ground from all the relative PIC inputs. And do add 10K resistors. Not more than that because of noise that can distrurb the input. Better change the 2K to 100 ohms too.

The usual practice is to have Pull Ups and then the opto drive to ground the input. If you want the input high then just reverse the byte your read from the port in software. Easy task.

Also note that in 886, there are Pull-Ups only on Port B and not on Port A.

Ioannis

[pescador]

Ok thanks - I'll connect the opto collector tied high to the PIC and drive the Opto to GND - easy - no need then for ground resistors...

Thanks for R values..REV.pdf

[Ioannis]

Maybe I was not very clear.

With your original design, you would change the 2K to 100 ohms.

But with the new design, the pullup will be around 10K. NOT 100 ohms. I am pretty sure you will destroy the optos.

Ioannis

[pescador]

- So just to be clear - I'm driving the optos using the 2k to the anode, the output is the "collector" tied hi to a resistor - to the PIC w 5V. I'm not sure 100 ohms to the PIC would damage the PIC, but I dont think the opto would care?

In any case - I'll use 10K like you suggest.. no real reason to use 100.

[Ioannis]

With 100 ohms from +5 to the collector of opto, you will force 50mA through opto. This is the absolute max according to the data sheet. It will lead sooner or later to the destruction of the opto.

Please do use 10K from +5 to the collector of the opto and drive from the collector the PIC port. The emitter of the opto will be earthed.

Everytime you feed current to the LED of the opto, the PIC port will be low and when the LED is off, port will be high.

Ioannis